The invention described herein was made by employees of the United States Government and may be used by or for the Government for governmental purposes without the payment of any royalties thereon or therefor.
1. Field of the Invention
The present invention is generally related to an apparatus and method for measuring the strain of optical fibers using Rayleigh scattered light.
2. Problem to be Solved
It is often desirable to measure optical fibers for strain that occurs when the optical fibers are subjected to tension or compression. Strain can occur if the optical fiber is subject to physical forces that stretch, contort or contract the optical fiber. Stress to the optical fibers can also occur as a result of variations in temperature of the environment within which the optical fiber is located. Such physical forces and temperature variations are typically found in aerospace environments such as aircraft or spacecraft.
Conventional systems and methods of measuring distributed strain in optical fibers have relied upon some alteration of the existing fiber. One conventional system uses extrinsic Fabry-Perot interferometers which require construction of small air gaps within the fiber. Another conventional system uses intrinsic Fabry-Perot interferometers which require the construction of two reflecting surfaces in the optical fiber generally by use of a fusion splice. Another conventional system utilizes Bragg gratings which are produced by UV (ultraviolet) exposure causing periodic changes in the index of refraction of the fiber core. Such alterations to the optical fiber are costly, time consuming and significantly weaken the fiber, making installation difficult and expensive. What is needed is an apparatus and method for measuring the strain in optical fibers that can use relatively inexpensive, mass produced and rugged commercial grade single mode optical fibers.
It is therefore an object of the present invention to provide a new and improved apparatus and method for measuring strain in optical fibers.
It is a further object of the present invention to provide a new and improved apparatus and method for measuring strain in optical fibers with a relatively high degree of accuracy.
It is yet another object of the present invention to provide a new and improved apparatus and method for measuring strain in optical fibers that may be implemented cost effectively.
Still other objects and advantages of the present invention will in part be obvious and will in part be apparent from the specification.
The above and other objects and advantages, which will be apparent to one of skill in the art, are achieved in the present invention which is directed to an apparatus and method for measuring strain in an optical fiber using the spectral shift of Rayleigh scattered light. The interference pattern produced by an air gap reflector and backscattered radiation is measured. Using Fourier Transforms, the spectrum of any section of fiber can be extracted. Cross correlation with a reference zero-load (i.e. zero-strain or zero-compression) measurement produces a correlation peak. The location of the correlation peak indicates the strain level in the selected portion of optical fiber.
In one aspect, the present invention is directed to a method for measuring the strain in an optical fiber comprising the steps of providing an optical radiation source configured to transmit optical radiation over a plurality of contiguous predetermined wavelength ranges, and an optical detector, splitting the optical radiation into a reference radiation portion and a measurement radiation portion, transmitting the measurement radiation portion into an optical fiber having non-zero Rayleigh scatter, collecting the backward Rayleigh scattered light, joining the reference and measurement radiation portions at the input of the optical detector in order to produce an interference fringe, measuring the interference fringe over a plurality of wavelengths to produce a reference pattern, measuring the interference fringe over a plurality of wavelengths after the optical fiber has been subjected to strain or compression so as to produce a measurement pattern, computing the Fourier Transform of the reference pattern as a function of wavenumber so as to produce a reference spatial domain response, computing the Fourier Transform of the measurement pattern as a function of wavenumber so as to produce a measurement spatial domain response, selecting the complex points associated with a segment of the measurement spatial domain response and a corresponding segment of the reference spatial domain response, performing an inverse Fourier Transform of the segment of the measurement spatial domain response to provide a first transformation, performing an inverse Fourier Transform of the segment of the reference spatial domain response to provide a second transformation, determining the complex cross-correlation between the first and second transformations, determining the amplitude of the cross-correlation as a function of wavenumber shift, and determining the wavenumber shift corresponding to a peak of the amplitude of the cross-correlation wherein such wavenumber shift indicates strain in the segment of the optical fiber that corresponds to the segments of the measurement and reference spatial domain responses.
In a related aspect, the present invention is directed to a method for measuring the strain in an optical fiber comprising the steps of providing an optical radiation source configured to transmit optical radiation over a plurality of contiguous predetermined wavelength ranges, and an optical detector, splitting the optical radiation into a reference radiation portion and a measurement radiation portion, transmitting the measurement radiation portion into an optical fiber having non-zero Rayleigh scatter, collecting the backward Rayleigh scattered light, joining the reference and measurement radiation portions at the input of the optical detector in order to produce an interference fringe, measuring the interference fringe over a plurality of wavelengths to produce a reference pattern, measuring the interference fringe over a plurality of wavelengths after the optical fiber has been subjected to strain or compression so as to produce a measurement pattern, computing the Fourier Transform of the reference pattern as a function of wavenumber so as to produce a reference spatial domain response, computing the Fourier Transform of the measurement pattern as a function of wavenumber so as to produce a measurement spatial domain response, selecting the complex points associated with a segment of the measurement spatial domain response and a corresponding segment of the reference spatial domain response, computing the complex conjugate of all complex points in the selected segments of the spatial domain responses of the reference and measurement patterns, computing the product of the complex conjugate of all complex points in the selected segment of the reference spatial domain responses and the complex conjugate of all complex points in the selected segment measurement patterns wherein the product comprises an array of product values, computing the inverse Fourier Transform of the array of product values, computing the amplitude of the array as a function of wavenumber shift, and determining the wavenumber shift corresponding to the peak of the amplitude of the array wherein the wavenumber shift indicates strain in the section of optical fiber that corresponds to the segments of the measurement and reference spatial domain responses.